Optical lithography is a key technology used in manufacturing of semiconductor devices, flat panel displays, and other devices. As feature dimensions are continuously being reduced to fit more components on each device, lithography is being pushed towards and beyond the classical resolution limit of optical imaging systems. The characteristics of the aerial image that is projected from a mask onto a substrate, e.g., a semiconductor wafer, are being manipulated by a wide range of resolution enhancement techniques (RET), such as phase shifting or optical proximity correction (OPC) in order to achieve the desired results. At the same time, with shrinking device dimensions, processing margins and process windows, which determine manufacturability and yield of a process, are being squeezed. More and better diagnostics are required to keep processes in control as well as to develop and improve numerical models that predict process capabilities and enable successful implementation of RET.
It has recently been demonstrated that the aerial image in a lithographic projection system can be directly measured with high spatial resolution in situ and under conditions that exactly match the conditions used in production. An image sensor unit having resolution-enhanced sensor elements and a method of aerial image acquisition are described in U.S. Pat. No. 6,828,542, “System and Method for Lithography Process Monitoring and Control,” the subject matter of which is hereby incorporated by reference in its entirety. Such an image sensor unit can be loaded onto the wafer stage of a lithographic projection system in place of a regular production semiconductor wafer and be repeatedly exposed with the projected image from a mask in the same way as a production wafer.
With reference to FIG. 1, in one embodiment of the invention disclosed in the U.S. Pat. No. 6,828,542, aerial image sensing system 100 includes lithographic equipment 10 (for example, a stepper), an image sensor unit 102, and a processor/controller 104, for example, a computer and/or data or image processing unit. Lithographic equipment 10 may include a mirror 12, a light source 14 to generate light 16 at a certain exposure wavelength, illumination optics 18, projection optics 20, and a chuck 22. Chuck 22 secures image sensor unit 102 in a fixed location using, for example, electrostatic or vacuum forces.
The optics of lithographic equipment 10 (for example, light source 14, illumination optics 18, and projection optics 20) interacts with a mask 26 to project an aerial image onto image sensor unit 102. Mask 26, in one embodiment of the invention disclosed in the U.S. Pat. No. 6,828,542, may be a product-type mask; that is, a mask used to form circuits during integrated circuit fabrication. As such, mask 26 contains the pattern to be replicated or printed on a wafer that ultimately contains the circuit design (or a portion thereof) of the integrated circuit. In this embodiment, image sensor unit 102 may be employed to evaluate the interaction between mask 26 and lithographic equipment 10 (whether production or non-production equipment) as well as characterize the performance of lithographic equipment 10 or the quality of mask 26.
In another embodiment of the invention disclosed in the U.S. Pat. No. 6,828,542 , mask 26 may be a test mask that is used to inspect, characterize and/or evaluate the optical characteristics or response of lithographic equipment 10. In this regard, mask 26 may include a fixed, predetermined and/or known pattern against which the aerial image collected, sensed, sampled, measured and/or detected by image sensor unit 102 will be evaluated, measured, and/or compared. In this way, any errors or discrepancies in the aerial images may be isolated or attributed to the optical system of lithographic equipment 10 and the performance of that system may be evaluated or characterized.
With continued reference to FIG. 1, image sensor unit 102 collects, measures, senses and/or detects the aerial image produced or generated by lithographic equipment 10 in conjunction with mask 26. Image sensor unit 102 provides image data, which is representative of the aerial image, to processor/controller 104. Processor/controller 104, in response, evaluates and/or analyzes that data to inspect, characterize and/or evaluate mask 26 and/or lithographic equipment 10 (or sub-systems thereof, for example, the optical sub-system). In this regard, processor/controller 104 implements data processing and analysis algorithms to process the data from image sensor unit 102 to reconstruct a full or partial aerial image, or to extract desired information directly without reconstructing a full or partial aerial image. Such image processing may involve deconvolution or other techniques familiar to those skilled in the art.
In addition, processor/controller 104 may use the data from image sensor unit 102 to perform and evaluate critical dimension measurements, and/or conduct defect inspection, for example, by comparing the measured aerial image to a pattern design database, or perform a die-to-die inspection if there are multiple dice on the same mask. Processor/controller 104 may also implement algorithms that conduct or perform resist modeling and/or integrated circuit yield analyses.
Processor/controller 104 may be employed as a control or operator console and data/image processing device. Processor/controller 104 may store algorithms and software that process the data representative of the aerial image (received from image sensor unit 102), extract information, manage data storage, and/or interface with users/operators. Processor/controller 104 may be located near or next to lithographic equipment 10 or in another locale, which is remote from lithographic equipment 10.
It should be noted that processor/controller 104 may be a stand-alone unit, as illustrated in FIG. 1, or partially or wholly integrated in lithographic equipment 10. In this regard, suitable circuitry in lithographic equipment 10 may perform, execute and/or accomplish the functions and/or operations of processor/controller 104 (for example, evaluation and/or analysis of the data representative of the aerial image collected, measured, sensed and/or detected at the wafer plane). Thus, in one embodiment, the inspection, characterization and/or evaluation circuitry/electronics may be partially or wholly integrated into lithographic equipment 10 and, as such, this “integrated system” may determine, assess, apply and/or implement appropriate corrective measures to enhance or improve its operation and thereby improve or enhance the quality, yield, and cost of integrated circuits manufactured therein.
It should be further noted that processor/controller 104 may also be partially or wholly integrated in, or on, image sensor unit 102. In this regard, some or all of the functions and operations to be performed by processor/controller 104 may be performed, executed and/or accomplished by suitable circuitry in, or on image sensor unit 102. As such, the collection and analysis of data representative of the aerial image may be less cumbersome in that a bus may be integrated and/or fabricated on or within image sensor unit 102 to facilitate communication of data and commands to/from the circuitry used to measure, detect and/or sense the aerial image and the circuitry used to evaluate and/or analyze the data representative of the aerial image.
With reference to FIG. 2, in one embodiment of the invention disclosed in the U.S. Pat. No. 6,828,542, an image sensor array 106 of image sensor unit 102 is shown. Image sensor array 106 includes a plurality of sensor elements 200, including 200ax to 200hx (x=1 to 8), that measure, sense, detect and/or collect incident energy or radiation.
In those instances where the dimensions of the active areas of sensor elements 200 are too large to provide a desired or required spatial resolution, it may be necessary to limit, restrict, and/or reduce these sensor cells' active areas that are exposed. Hence, image sensor array 106 may include a patterned opaque film 204 that impedes, obstructs, absorbs, and/or blocks passage of photons or light of a given wavelength (that is, at the wavelength to be measured, sensed or detected by sensor elements 200). Opaque film 204 includes apertures 206, including 206ax to 206hx (x=1 to 8), so that active areas of sensor elements 200 are exposed only at apertures 206. As such, the spatial resolution of the energy measured by sensor elements 200 is enhanced or improved because the portion or area of each sensor element that is effectively exposed to and/or measures, senses, detects, and/or collects energy or radiation is limited or restricted.
Generally, sensor elements 200 as well as any resolution enhancing measures, e.g., small apertures 206 formed in a light-blocking layer on top of sensor elements 200, will be arranged on a very regular 2-dimensional grid. As also described in the U.S. Pat. No. 6,828,542, the aerial image of a projection apparatus can be sampled and reconstructed by loading image sensor unit 102 into the image plane of the projection optics, exposing image sensor unit 102, and subsequently introducing small lateral shifts in x and y directions of image sensor unit 102 relative to the aerial image before further iterated exposures. This data acquisition scheme, which resembles the operation principle of a scanning imaging microscope, is illustrated in FIG. 3A. This figure depicts schematically a two-by-two subset 300 of a larger 2D (two-dimensional) image sensor array (such as image sensor array 106 shown in FIG. 2). The black dot close to the center of each light-sensing element 302 indicates a resolution-enhancing element, e.g., a small aperture 301 that enables sufficient discrimination of the smallest spatial features in the aerial image. Also, the projected images 303 are schematically indicated as feature shapes of the aerial image that is projected onto light-sensing elements 302. Small open circles 304 indicate subsequent sampling locations that result from small, programmed lateral steps 305 in between repeated exposures of the light-sensing element 302. From these subsequent exposures and the data recorded by each individual light-sensing element 302, a small area of the projected image 303 can be reconstructed with a spatial resolution that depends on the size of apertures 301 and/or the step size 305 in between exposures. The reconstructed high-resolution image “patches” of various light-sensing elements 302 are independent so that each light-sensing element 302 may sample different, arbitrary image features. A complete reconstruction of the full mask image is, in principle, possible by capturing a sufficiently large number of exposures and by covering an area with each light-sensing element 302 that matches or exceeds the lateral light-sensing element “grid period” in x and y directions (i.e., the width and height, respectively, of each light-sensing element 302). Thus, in principle, detailed investigation of completely arbitrary mask layouts is possible, as is detection of single, isolated image defects.
However, to achieve high resolution of the reconstructed image, small lateral steps and therefore a very large number of successive exposures are required if a substantial fraction of the complete mask area is to be inspected. Typical grid periods for imaging sensor arrays are on the order of 10 um, while spatial dimensions on leading edge semiconductor layouts are below 0.1 um. Therefore, for complete “mask inspection” at high resolution, at least on the order of 10,000 exposures would be required. This requirement implies relatively long data acquisition times, during which the projection tool would be unavailable for production. In addition, certain drifts or performance changes over time may occur during data acquisition periods, for example, due to lens heating effects.
There are many situations in which it may be beneficial to assess the overall quality of the aerial image in a projection system, while it may not be required to inspect individual mask features or detect individual point defects. In such situations there is a clear need for generating relevant information in as short a time as possible in order to maximize availability of the lithography tool for production. Such situations include investigations of aerial image quality in response to effects or adjustments that are expected to result in global variations, e.g., of image contrast, or spatially slowly varying changes within the exposure field of the projection apparatus. Some examples of parameters or parameter changes that result in global image variations are focus position, focal plane tilt and shape, low-order aberrations, flare, measurements of image contrast vs. pitch, and line width. Also, in process development or design rule verification, investigating image quality of particular features on average may provide more useful information than assessing an individual instance of a particular feature.
Another challenge may arise from the fact that the smallest aerial image features in a leading-edge lithographic projection system are often smaller than the wavelength used to project the image from a mask onto a wafer. Resolving such small features with a scanning aperture generally requires an aperture size smaller or at least comparable to that of the smallest feature sizes. The optical intensity transmitted through an aperture comparable or smaller than the wavelength of the radiation is typically very low and may limit the signal-to-noise ratio (SNR) of the detected signal. In order to improve the SNR, an aerial image sensor that had an array of many apertures formed on a single photo detector (i.e., a photodiode) was manufactured in a prior art system; this aerial image sensor was also stepped sequentially across the wafer plane in a lithographic projection system in order to reconstruct a high-resolution aerial image. See “A Two-Dimensional High-Resolution Stepper Image Monitor,” A. K. Pfau et al., Proc. SPIE Vol. 1674 (1992). FIG. 3B illustrates this prior art system. Here, multiple apertures 306 are on one light-sensing element 307 for capturing the projected images 308. Small open circles 309 indicate subsequent sampling locations that result from small, programmed lateral steps 310 in between repeated exposures of the single light-sensing element 307.
Since the signal measured by the photo detector is the sum of intensities transmitted through the individual apertures, a requirement for image reconstruction is to provide an image with regularly spaced, identical, and repeated features on a grid that exactly matches the aperture grid. While improving the SNR, this arrangement puts very stringent limitations on the mask design layout and essentially prevents inspection of arbitrary, production-type photo masks.
Both methods illustrated in FIGS. 3A and 3B rely on stepping apertures across the projected image at very well-controlled, known positions in order to reconstruct an undistorted high-resolution image. While overlay accuracies in the nanometer range are currently achieved in high-end lithographic projection tools, the requirement for very precise and repeatable alignment of the sensor with respect to the projected image over a potentially very large number of subsequent exposures is clearly a technical challenge, and any imperfections in this process may introduce additional noise or distortions to the reconstructed aerial image.